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You need a metal to oxidise, Zinc does this quite well with a good potential.
I've been collecting wheel-balancing weights (you find 'em by the kerb) a lot of these are zinc (pic) - otherwise see other comments.
L

Yes, carbon rods, would work, but you would get less voltage/current because of the increased resistance of the carbon compared to the zinc(and I think the difference of electronegativity) I tried it: With carbon/copper, I got 0.7V, and with copper/zinc, I got 1V. Zinc can be commonly found on so-called "hot galvanized" nails : they are covered in Zinc for better rust resistance.
Good Luck!